Rectifiers are electrical devices that are particularly adapted to rectifying current, that is converting alternating current to direct current. More specifically, rectifiers exhibit a very low resistance to current flow when forward-biased (i.e., anode biased more positive than cathode) and a very high resistance to current flow when reverse-biased (i.e., anode biased more negative than cathode).
One known form of rectifier is a semiconductor p-i-n diode, which typically comprises semiconductor layers arranged as P.sup.+ /N/N.sup.+. The "P.sup.+ " and "N.sup.+ " layers constitute semiconductor regions that are highly-doped with P-conductivity type dopant and with N-conductivity type dopant, respectively. The intermediate "N" layer is relatively lightly doped with N-conductivity type dopant so that it can support high reverse voltages without current conduction.
In operation of a p-i-n diode, a forward bias to typically 0.8 to 1.0 volts (for silicon devices) is required to initiate current conduction. This forward voltage drop of 0.8 to 1.0 volts undesirably results in a high level of waste heat generation during forward conduction; a rectifier with a lower forward voltage drop would thus be desirable so as to limit waste heat generation.
A p-i-n diode is a "bipolar" device in that current flow in the diode is due to current carriers of both types, that is, both holes and electrons. Relative to unipolar device in which current flow is due to only hole or electron flow, bipolar devices are slow at turning off, since, after turn-off initiation in a bipolar device, there is a delay during which minority carriers (i.e., holes in the "N" region of a typical p-i-n diode) recombine with majority carriers. The slower turn-off speed of bipolar device make them less suitable than unipolar devices for high-speed switching applications.
A rectifier that was developed to provide a lower forward voltage drop and a faster turn-off speed than a p-i-n diode is the Schottky diode. In a typical Schottky diode, a Schottky barrier contact is formed between a first electrode and a first N-conductivity type layer of semiconductor material. The first layer has a dopant concentration per cubic centimeter below about 1.times.10.sup.17, at least for N-conductivity type silicon. A Schottky barrier contact exhibits a potential barrier to current flow and, like a p-i-n diode, must be forward biased to initiate current flow. If the first layer had a dopant concentration in excess of the foregoing value, an ohmic contact between the first electrode and first layer would result, which does not exhibit a potential barrier to current flow. In the foregoing Schottky diode, an ohmic contact is formed between a second N-conductivity type layer of more highly doped semiconductor material adjoining the first layer and a second electrode.
Although Schottky diodes exhibit lower forward voltage drops and faster turn-off speeds than p-i-n diodes, this is at the expense of exhibiting high reverse leakage currents, which increase significantly for increasing values of reverse voltage.
A rectifier which attains a low forward voltage drop and fast turn-off speed without exhibiting a high level of reverse leakage current is described in commonly assigned U.S. Pat. No. 4,641,174, which is incorporated herein in its entirety by reference. The rectifier described in the above-identified patent generally is referred to as a pinch rectifier. In the pinch rectifier, conduction channel regions are formed in the drift region, and the forward resistance to current flow of the pinch rectifier is a function of, among other things, the length of the drift region. Specifically, as the length of the drift region is shortened, the forward resistance of the pinch rectifier decreases.
Further, in the pinch rectifier disclosed in the above-identified patent, spacing of P.sup.+ regions disposed adjoining the upper portion of the drift region determines when reverse current flow will be interrupted. Specifically, as the P.sup.+ regions are disposed closer to each other, depletion regions induced in the drift region by the P.sup.+ regions merge faster, thereby providing faster interruption of reverse current flow. Under the latter condition, however, the forward resistance of the rectifier increases and results in an undesirable increase in the threshold forward-bias voltage.